Free machining steel for machine structural use having improved chip disposability

ABSTRACT

Disclosed is a free machining steel for machine structural use, which can realize very good chip disposability without incorporation of any harmful material such as lead (Pb). This free machining steel is a hot rolled or hot forged free machining steel having improved chip disposability. The free machining steel comprises, by mass, carbon (C): 0.01 to 0.70%, silicon (Si): 0.05 to 2.00%, manganese (Mn): 0.20 to 3.50%, calcium (Ca): 0.0003 to 0.01%, sulfur (S): 0.020 to 0.300%, aluminum (Al): 0.002 to 0.300%, nitrogen (N): 0.003 to 0.035%, and oxygen (O): 0.0010 to 0.0080% with the balance consisting of iron (Fe) and unavoidable impurities. The steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides. The steel also comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm 2  of the inspection area.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a free machining steel for machine structural use, which contains various free machining materials for machining cost reduction purposes, and more particularly to a free machining steel for components including automobile components.

[0003] 2. Background Art

[0004] In steels for machine structural use including automobile components, various free machining materials have hitherto been incorporated for machining cost reduction purposes. Representative examples of such free machining steels include leaded free machining steels, resulfurized free machining steels, calcium-deoxidized free machining steels, and complex free machining steels produced by using the above free machining materials in combination.

[0005] Among them, leaded free machining steels are most commonly used, because, as compared with fundamental steels, the leaded free machining steels are less likely to cause a deterioration in mechanical properties and is superior in the effect of improving machinability, particularly tool life or chip disposability during low-speed machining. A growing concern about environmental problems in recent years, however, has led to a worldwide tendency toward a reduction in the amount of lead, which is harmful to the human body, used in steels. This has in turn led to an increasing demand for the development of free machining steels as an alternative to the leaded free machining steels.

[0006] Resulfurized free machining steels can be considered as one of free machining steels usable as an alternative to the leaded free machining steels. In resulfurized free machining steels, however, since sulfur is present in steel as MnS inclusions which are elongated in a rolling direction, the addition of a large amount of sulfur disadvantageously increases the anisotropy of mechanical properties.

[0007] Calcium-deoxidized free machining steels are steels which have been improved in machinability by incorporating a low-melting CaO—Al₁O₃—SiO₂ oxide in the steel. This machinability is realized through such a mechanism that the above oxide forms, at the edge of the tool, a protective film which prevents direct contact between chips and the tool. In the calcium-deoxidized free machining steels, however, this effect can be attained only during relatively high-speed machining such as sintered carbide tool machining.

[0008] Leaded ternary free machining steels containing all of lead, sulfur, and calcium in a combined form are also used. The leaded ternary free machining steels have very good machinability, but on the other hand, the above-described drawbacks involved in leaded free machining steels and resulfurized free machining steels cannot be alleviated.

[0009] Japanese Patent No. 1981560, Japanese Patent Laid-Open No. 350065/1999, and Japanese Patent Laid-Open No. 34538/2000 disclose steels in which calcium has been incorporated for improving mechanical properties of the resulfurized free machining steel. Further, in this case, the conversion of hard Al₂O₃ to CaO—Al₂O₃ or the covering of Al₂O₃ with a sulfide is reported to render.the steel harmless. Japanese Patent Laid-Open No. 145889/1994 discloses steels which have been improved in machinability by incorporating hexagonal BN, CaO—Al₂O₃, and Ca—Mn—S. In these cases, regarding the anisotropy of mechanical properties, as compared with the fundamental steel, the deterioration level can be reduced by controlling the form of sulfides. Regarding the machinability, however, satisfactory results are not always obtained under various machining conditions. At the present time, particularly for chip disposability which is the most important property of steels used as the alternative to the leaded free machining steels, the chip disposability level does not always reach that of the leaded steels.

[0010] Japanese Patent Laid-Open No. 180184/2002 discloses structural steels which have been improved in machinability, mainly service life of drills, and anisotropy of strength by regulating the size of sulfides. Regarding the chip disposability, however, the proposed structural steels are far from steels having satisfactory properties which are usable as steels alternative to the leaded free machining steels.

SUMMARY OF THE INVENTION

[0011] The present inventor has now found that, in hot rolled or hot forged steels having a predetermined composition, very good chip disposability can be realized, without incorporating harmful materials such as lead, by regulating the number of sulfides composed mainly of MnS having a predetermined size and the number of oxide inclusions having a predetermined size.

[0012] Accordingly, an object of the present invention is to provide a free machining steel for machine structural use, which can realize very good chip disposability without incorporation of any harmful material such as lead.

[0013] According to the present invention, there is provided a free machining steel having improved chip disposability, wherein the steel is hot rolled or hot forged and comprises, by mass,

[0014] carbon (C): 0.01 to 0.70%,

[0015] silicon (Si): 0.05 to 2.00%,

[0016] manganese (Mn): 0.20 to 3.50%,

[0017] calcium (Ca): 0.0003 to 0.01%,

[0018] sulfur (S): 0.020 to 0.300%,

[0019] aluminum (Al): 0.002 to 0.300%,

[0020] nitrogen (N): 0.003 to 0.035%, and

[0021] oxygen (O): 0.0010 to 0.0080%,

[0022] the balance consisting of iron (Fe) and unavoidable impurities,

[0023] wherein the steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides, and

[0024] wherein the steel comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm² of the inspection area.

[0025] In a preferred embodiment of the present invention, the free machining steel according to the present invention further comprises one or more members selected from, by mass, chromium (Cr): 0.20 to 2.50%, molybdenum (Mo): 0.05 to 1.50%, nickel (Ni): 0.05 to 3.50%, vanadium (V): 0.01 to 0.50%, niobium (Nb): 0.01 to 0.10%, and titanium (Ti): 0.01 to 0.50%.

[0026] In a preferred embodiment of the present invention, the free machining steel according to the present invention further comprises one or more members selected from, by mass, magnesium (Mg): 0.0003 to 0.01%, zirconium (Zr): 0.0005 to 0.30%, bismuth (Bi): 0.01 to 0.30%, and boron (B): 0.0003 to 0.015%.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The reasons for the limitation of individual constituents in the free machining steel for machine structural use having improved chip disposability according to the present invention will be described. In the chemical composition of the steel, “%” is by mass.

[0028] Carbon (C) is an element that is added for ensuring the strength of the steel. When the content of carbon is less than 0.01%, the strength of the steel is unsatisfactory. On the other hand, when the carbon content exceeds 0.70%, the toughness of the steel is deteriorated. For this reason, the carbon content of the steel according to the present invention is limited to 0.01 to 0.70%.

[0029] Silicon (Si) is an element that is added for deoxidation during steelmaking and for ensuring the strength of the steel. When the content of silicon is less than 0.05%, the deoxidation effect cannot be satisfactorily attained. On the other hand, when the silicon content exceeds 2.00%, the hot workability is deteriorated. For the above reason, the silicon content of the steel according to the present invention is limited to 0.05 to 2.00%.

[0030] Manganese (Mn) is an element that is added for improving hardenability. Further, manganese is an indispensable element that, together with sulfur, forms a sulfide for improving machinability. Furthermore, MnS is also effective in suppressing coarsening of austenite grains to refine the texture. When the content of manganese is less than 0.20%, however, the contemplated effect is small. On the other hand, when the manganese content exceeds 3.50%, the workability is deteriorated. Therefore, the manganese content is limited to 0.20 to 3.50%.

[0031] Calcium (Ca) has the effect of improving the anisotropy through the control of the form of sulfides. Further, calcium can improve the chip disposability. Furthermore, calcium can deposit a protective film of (Mn, Ca)S and AIN on a tool to prolong the service life of the tool. In addition, calcium can form a Ca oxide to improve chip disposability. The above effect can be attained by the addition of calcium in an amount of not less than 0.0003%, preferably not less than 0.001%. When the content of calcium exceeds 0.01%, the effect is saturated. In this case, further, the yield of addition of calcium is lowered. For the above reason, the content of calcium in the steel according to the present invention is limited to 0.0003 to 0.01%.

[0032] Sulfur (S) has the effect of forming sulfides such as MnS and (Mn, Ca)S to improve machinability. When the steel is heated to 1000° C. or above for hot working, these sulfides can suppress coarsening of austenite grains. When the steel according to the present invention is used as a microalloyed steel, sulfur can advantageously enhance the toughness of the steel. In order to attain the above effects, the content of sulfur should be not less than 0.020%, preferably not less than 0.050%. When the content of sulfur exceeds 0.300%, however, the toughness is deteriorated due to stress concentration effect of sulfides. Therefore, in the steel according to the present invention, the sulfur content is limited to 0.020 to 0.300%.

[0033] Aluminum (Al), as with silicon, is added for deoxidation during steelmaking. The composite oxide produced as a result of the deoxidation effectively contributes to chip disposability. Further, aluminum forms AIN which contributes to the refinement of austenite grains. In order to attain the above effects, the content of aluminum should be not less than 0.002%. The addition of aluminum in an amount of more than 0.300% results in the formation of aluminum oxide which deteriorates toughness and machinability of the steel. For the above reason, in the steel according to the present invention, the content of aluminum is limited to 0.002 to 0.300%, preferably 0.003 to 0.015%.

[0034] Nitrogen (N) is added for increasing the toughness of the steel. Further, nitrogen forms nitrides such as AIN which can effectively refine austenite grains. In order to attain the above effect, the content of nitrogen should be not less than 0.003%. When the amount of nitrogen added is more than 0.035%, the effect is saturated. For the above reason, in the steel according to the present invention, the content of nitrogen is limited to 0.003 to 0.035%, preferably 0.005 to 0.020%.

[0035] Oxygen (O) forms an oxide which is effective for ensuring machinability and further serves as nuclei of sulfides for fine dispersion. In order to attain the above effect, the content of oxygen should be not less than 0.0010%. When the oxygen content is more than 0.0080%, mechanical properties are deteriorated. Therefore, in the steel according to the present invention, the oxygen content is limited to 0.0010 to 0.0080%, preferably 0.0015 to 0.0050%.

[0036] In a preferred embodiment of the present invention, the steel according to the present invention further contains 0.20 to 2.50% of chromium (Cr). Chromium functions similarly to manganese. That is, chromium is an element that enhances hardenability and improves strength. When the content of chromium is less than 0.20%, the contemplated effect is small. On the other hand, when the chromium content is more than 2.50%, the cost is increased.

[0037] In a preferred embodiment of the present invention, the steel according to the present invention further contains 0.05 to 1.50% of molybdenum (Mo). Molybdenum functions similarly to chromium. That is, molybdenum is an element that enhances hardenability and improves strength. When the content of molybdenum is less than 0.05%, the contemplated effect is small. On the other hand, when the molybdenum content is more than 1.50%, the cost is increased.

[0038] In a preferred embodiment of the present invention, the steel according to the present invention further contains 0.05 to 3.50% of nickel (Ni). Nickel functions similarly to molybdenum. That is, nickel is an element that enhances hardenability and improves strength. When the content of nickel is less than 0.05%, the contemplated effect is small. On the other hand, when the nickel content is more than 3.50%, the cost is increased.

[0039] In a preferred embodiment of the present invention, the steel according to the present invention further contains 0.01 to 0.50% of vanadium, 0.01 to 0.10% of niobium and/or 0.01 to 0.50% of titanium. Vanadium, niobium, and titanium form fine carbonitrides in the steel, and these precipitates refine austenite grains during hot working and improve the toughness of the steel. Further, the effect of improving the strength of the steel can be attained through dispersion strengthening or precipitation strengthening of these precipitates. For all of vanadium, niobium, and titanium, when the content is not more than 0.01%, this effect cannot be attained. When the content is excessively large, the toughness is reduced. For the above reason, the upper limit of the vanadium content is 0.50%, the upper limit of the niobium content is 0.10%, and the upper limit of the titanium content is 0.50%.

[0040] In a preferred embodiment of the present invention, the steel according to the present invention further contains 0.0003 to 0.01% of magnesium, 0.0005 to 0.30% of zirconium, 0.01 to 0.30% of bismuth, and/or 0.0003 to 0.015% of boron. As with calcium, magnesium and zirconium are present as a sulfide or an oxide, bismuth is present either solely or together with other inclusions, and boron is present as a nitride. These elements, which are present in the above respective forms, further improve the chip disposability of the steel according to the present invention. Magnesium and zirconium are each an element that functions to control the form of sulfides and further has the effect of improving mechanical anisotropy. When the magnesium content is less than 0.0003%, when the zirconium content is less than 0.0005%, when the bismuth content is less than 0.01%, and when the boron content is less than 0.0003%, the above effects are unsatisfactory. On the other hand, when the magnesium content is more than 0.01%, when the zirconium content is more than 0.30%, when the bismuth content is more than 0.30%, and when the boron content is more than 0.015%, the above effects are saturated, resulting in increased cost.

[0041] The free machining steel according to the present invention is a hot rolled or hot forged free machining steel having the above chemical composition, wherein the steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides, and wherein the steel comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm² of the inspection area. The above constitution can provide a free machining steel for machine structural use, which has mechanical properties substantially comparable to leaded free machining steels, has good machinability, and particularly has chip disposability comparable to leaded steels.

[0042] It is considered that this machinability improvement effect can be attained through the following mechanism. However, it should be noted that the following description is based on a hypothesis and the present invention is not limited to this hypothesis. At the outset, when the free machining steel for machine structural use according to the present invention is machined, cracking is likely to occur around inclusions composed mainly of MnS having a relatively large size of over 20 μm in major axis. When a large number of relatively fine sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, (which occupy not less than 30% of all the sulfides) are present in the vicinity of the cracks, these sulfides effectively accelerate the propagation of cracks of sulfides. In this case, the effect attained by sulfides only is small. However, when oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, are present in an amount of not less than 10 in number per mm² of the inspection area, crack propagation in chips is significantly accelerated, resulting in improved chip disposability.

[0043] The free machining steel according to the present invention can be produced as follows. Specifically, in a steelmaking process, in addition to conventional regulation of chemical composition, the dissolved oxygen content is regulated to a value between 15 ppm and 90 ppm by deoxidation with aluminum and silicon, and then calcium is added. Thereafter, the steel is subjected to hot rolling or hot forging. Thus, there is obtained a steel product having the above chemical composition, wherein the steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides, and wherein the steel comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm² of the inspection area.

[0044] In a preferred embodiment of the present invention, the oxides in the steel according to the present invention may include CaO and Al₂O₃. Further, for example, MgO, MnO, SiO₂, TiO₂, ZrO₂, and REM oxides, which are produced in the presence of impurity elements or alloying elements having a high oxide forming capability, are present in combination with the above oxides.

EXAMPLES

[0045] The following Examples further illustrate the present invention but are not intended to limit it.

[0046] 100 kg of each of steel Nos. 1 to 20 having chemical compositions shown in Table 1 was produced by the melt process in a vacuum melting furnace. These steel ingots produced by the conventional process were subjected to forging at 1200° C. into steel bars having a diameter of 45 mm. TABLE 1 (mass %) No. Classification C Si Mn Ca S Al N O Others 1 Steel of invention 0.42 0.14 0.82 0.0023 0.058 0.005 0.012 0.0018 — 2 0.35 0.25 2.14 0.0030 0.030 0.024 0.016 0.0033 Mg: 0.0012 3 0.44 0.08 0.97 0.0012 0.082 0.015 0.008 0.0049 Cr: 1.15 4 0.20 0.26 0.81 0.0056 0.061 0.009 0.015 0.0012 Mo: 1.25 5 0.53 0.39 0.46 0.0045 0.132 0.008 0.010 0.0029 V: 0.25 6 0.41 0.22 0.92 0.0019 0.055 0.002 0.013 0.0052 Nb: 0.03 7 0.21 0.10 0.89 0.0026 0.034 0.017 0.004 0.0019 Ti: 0.11 8 0.16 0.23 2.03 0.0028 0.068 0.025 0.012 0.0025 Zr: 0.045 9 0.14 0.35 2.21 0.0011 0.081 0.009 0.019 0.0018 Mg: 0.005 10 0.15 0.23 1.47 0.0012 0.120 0.026 0.013 0.0033 Zr: 0.030 11 0.45 0.75 1.09 0.0076 0.103 0.015 0.006 0.0048 Bi: 0.10 12 0.44 0.31 0.98 0.0031 0.072 0.020 0.009 0.0055 Ni: 0.28 13 0.48 0.42 0.87 0.0022 0.096 0.019 0.022 0.0068 B: 0.0013 14 Comparative steel 1.02 0.25 0.76 0.0035 0.071 0.023 0.014 0.0010 — 15 0.05 1.39 3.61 0.0029 0.032 0.012 0.009 0.0028 — 16 0.21 0.56 1.59 0.0001 0.057 0.588 0.007 0.0015 Pb: 0.06 17 0.43 0.32 0.93 0.0018 0.120 0.015 0.020 0.0005 Mo: 2.2 18 0.54 0.26 0.62 0.0098 0.026 0.025 0.011 0.0033 Ti: 0.66 19 0.47 0.28 0.88 0.0037 0.005 0.021 0.016 0.0026 Pb: 0.15 20 0.33 2.11 0.68 0.0014 0.023 0.004 0.048 0.0031 —

[0047] In Table 1, constituents outside the scope of the present invention in the comparative steels are underlined.

[0048] Thereafter, the forged steel bars were quenched and tempered to regulate the hardness of all the steels to 25±2 HRC.

[0049] The steels thus obtained were subjected to the following measurements 1 to 3.

[0050] Measurement 1

[0051] The test steel was specularly polished, and inclusions having a size of 1 mm square were then photographed with an optical microscope at a magnification of 400 times. The major axis and number of all sulfides in 1 mm² were measured with an image analyzer. The number of small sulfides composed mainly of MnS having a major axis of not less than 0.5 μm and less than 20 μm was extracted from the results. The proportion of the number of small sulfides was calculated according to the following equation.

[0052] (Proportion of number of small sulfides)=(number of small sulfides/total number of sulfides)×100

[0053] Measurement 2

[0054] Optical photomicrographs of inclusions having a size of 1 mm square were provided to distinguish oxides from sulfides composed mainly of MnS by taking advantage of the difference in color tone between inclusions, and the number of oxides having a size of not less than 0.5 μm and less than 50 μm was measured with an image processor.

[0055] Measurement 3

[0056] The test steel was turned with a sintered carbide tool for machining made of JIS P 20 (edge R=0.4 mm) under the following conditions.

[0057] Cutting speed: 150 m/min

[0058] Feed rate: 0.1 mm/rev

[0059] Depth of cut: 0.5 mm

[0060] In the turning operation, chips were collected, and the number of chips per g (chip disposability index) was visually counted as an evaluation index for chip disposability.

[0061] The results of measurements 1 to 3 obtained for each steel were as shown in Table 2. TABLE 2 Classifi- Number of small Total number of Proportion of number of Number of oxides Chip disposability No. cation sulfides (per mm²) sulfides (per mm²) small sulfides (%) (per mm²) index* (chips per g) 1 Steel of 120 258 46.5 75 18 2 invention 56 184 30.4 89 23 3 211 674 31.3 176  25 4 328 787 41.7 32 21 5 453 598 75.8 55 24 6 449 864 52.0 76 32 7 235 457 51.4 246  35 8 567 891 63.6 91 32 9 514 690 74.5 123  41 10 1398 1758 79.5 110  25 11 1096 1897 57.8 564  36 12 435 576 75.5 741  43 13 512 789 64.9 905  27 14 Comp. 68 346 19.7 12  8 15 steel 251 576 43.6 23  9 16 234 467 50.1  8 25 17 689 981 70.2  4  1 18 35 167 21.0 34  2 19 21 98 21.4 25 31 20 123 244 50.4 51  7

[0062] Steels 1 to 13 of invention fall within the scope of the present invention. The proportion of small sulfides present in the steel according to the present invention is not less than 30%, and the number of oxides is not less than 10/mm².

[0063] For the steels of the present invention, the chip disposability index was at least 10 chips/g, whereas, for the comparative materials, i.e., Nos. 14, 15, 17, 18, and 20, the chip disposability index was less than 10 chips/g. For Nos. 16 and 19, the chip disposability index was not less than 10 chips/g. This is attributable to the fact that Nos. 16 and 19 contained lead, a harmful material.

[0064] As described above, in the free machining steel for machine structural use according to the present invention, very good chip disposability can be realized without incorporation of a harmful material such as lead. 

1. A free machining steel having improved chip disposability, wherein the steel is hot rolled or hot forged and comprises, by mass, carbon (C): 0.01 to 0.70%, silicon (Si): 0.05 to 2.00%, manganese (Mn): 0.20 to 3.50%, calcium (Ca): 0.0003 to 0.01%, sulfur (S): 0.020 to 0.300%, aluminum (Al): 0.002 to 0.300%, nitrogen (N): 0.003 to 0.035%, and oxygen (O): 0.0010 to 0.0080%, the balance consisting of iron (Fe) and unavoidable impurities, wherein the steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides, and wherein the steel comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm² of the inspection area.
 2. The free machining steel according to claim 1, which further comprises one or more members selected from, by mass, chromium (Cr): 0.20 to 2.50%, molybdenum (Mo): 0.05 to 1.50%, nickel (Ni): 0.05 to 3.50%, vanadium (V): 0.01 to 0.50%, niobium (Nb): 0.01 to 0.10%, and titanium (Ti): 0.01 to 0.50%.
 3. The free machining steel according to claim 1 or 2, which further comprises one or more members selected from, by mass, magnesium (Mg): 0.0003 to 0.01%, zirconium (Zr): 0.0005 to 0.30%, bismuth (Bi): 0.01 to 0.30%, and boron (B): 0.0003 to 0.015%. 